The invention relates to a method of and arrangement for producing coherent pure mode radiation modulated at a high bit rate, having two optically coupled injection lasers which may be controlled independently of one another.
For optical transmission of information via glass fibre light waveguides, semiconductor injection lasers are considered to be particularly suitable. Besides a series of other advantages, injection lasers have a direct modulability up to very high bit rates. For optical transmission of information via long monomode light waveguides at gigabit rates not only an appropriately rapid modulability is necessary, but also a narrow line of emission of the laser. If the laser is not pure mode, then considerable distortion of the propagation time occurs on monomode light waveguides, which distortion makes transmission of information at gigabit rates impossible over several km line length.
For rapid direct modulation, only those injection lasers can be considered which are biased above the threshold, since otherwise a delay of some nanoseconds occurs with each modulation pulse between the electrical pulse supplied to the laser and the optical output pulse. If the laser is biased above the threshold, then there is no switch on delay time, with a sudden change in the injection current the light intensity only adjusts to a new level, however with damped natural oscillations. In the interest of a rapid digital modulability, these natural oscillations should be very heavily damped. Decisive for the damping of the natural oscillations is the proportion of spontaneous emission in the emission of the oscillating modes. With monomode lasers, the proportion of spontaneous emission is very low and the natural oscillations decay only very slowly. In lasers in which a larger proportion of modes are energized, the spontaneous emission plays a greater part. The more rapid transient behaviour caused thereby acts positively; these lasers are also unsuitable however because of their wide spectrum. For largely physical reasons it appears therefore impossible to use a conventional injection laser as a directly modulatable optical transmitter for the highest bit rates.